Travelling wave tube having a graphite coating in the central region and the free end at least 10 wavelengths long and a qc of at least 0.4



Aug. 7. 1968 J. A. B. DECHERING ET AL 3,399,326

TRAVELLING wAvE TUBE HAVING A GRAPHITE COATING IN THE CENTRAL REGION ANDTHE FREE ENDS AT LEAST WAVELENGTHS LONG AND A QC OF AT LEAST 0.4 FiledSept. 10, 1964 5 Sheets-Sheet 1 GRAPHITE COATING dB/cm INVENTION PRIORART T I I I I I I I I I JOHANNE 5 AB. DEEHEWG JACOB OBER ANNE MEIJERAct? Aug. 27, 1 968 TRAVELLING WAVE TUBE HAVING A GRAPHITE COATING INTHE CENTRAL REGION AND THE FREE ENDS AT LEAST WAVELENGTHS- LONG AND A QCOF AT LEAST 0 4 Filed Sept. 10, 1964 J A. B. DECHERING ET AL 3Sheets-Sheet 2 G 50dB Q6 43 40 36 33 30 26 23 20 dB o I I I I I 1 I I l"0,01 0,05 0,2 5 20 100 500 Lwa 10% Aug. 27, 1968 1 cH l ETAL 3,399,326

TRAVELLING WAVE TUBE HAVING A GRAPHITE COATING IN THE CENTRAL REGION ANDTHE FREE ENDS AT LEAST 10 WAVELENGTHS LONG AND A QC OF AT LEAST 0.4

Filedsept. 10, 1964 3 Sheets-Sheet 5.0 kMc/s United States PatentTRAVELLING WAVE TUBE HAVING A GRAPHITE COATING IN THE CENTRAL REGION ANDTHE FREE ENDS AT LEAST WAVELENGTHS LONG AND A QC OF AT LEAST 0.4

Johannes Antonius Bernardus Dechering, Jacob Ober, and

Anne Meijer, Emmasingel, Eindhoven, Netherlands, assignors to NorthAmerican Philips Company, Inc., New York, N.Y., a corporation ofDelaware Filed Sept. 10, 1964, Ser. No. 395,383 2 Claims. (Cl. 315-35)ABSTRACT OF THE DISCLOSURE A travelling wave tube capable of amplifyingan electrical signal substantially without amplitude distortion and'A.M. to RM. conversionhaving attenuating material distributed about thecenter portion of the slow wave helix while portions at both ends arefree of attenuating material. The attenuation per unit length at thecenter portion is constant while the length of the output portion freeof attenuating material is at least ten wavelengths the value of the QCfor the tube being at least 0.4.

This invention relates to devices including a travelling wave tube inwhich attenuating material is provided on or near the helical delay lineso that portions at both ends are free from attenuating material. Italso relates to travelling wave tubes intended for such devices.

In travelling wave tubes of the aforementioned kind, the attenuatingmaterial may be provided in various ways. For example, a very highattenuation per unit length over a small portion of the length at acertain distance from the output; a comparatively low attenu ation overa large portion of the length at some distance from the 'output,preceded by a short length of high attenuation at the input end; a lowattenuation over a large portion of the length or an attenuation whichlinearly increases from the output to the input end.

In known arrangements, the first of which is regarded as unfavorable,the minimum distance of the attenuation from the output depends on theproduct QC for the relevant tube. The gain factor C, which is a measureof the coupling between the delay line and the beam, is defined inFormula 2.43 in the book Travelling-Wave Tubes (New York, 1950) by I. R.Pierce.'Q is the spacecharge parameter given by the Formula 7.15 on page113 in the above-mentioned book by I. R. Pience. In all these cases thelocation of the attenuation between the free portions at the ends isdetermined inter alia by the requirement that reflections at the outputmust not cause interfering variations in the group delay times.

An object of the invention is to provide a location for the attenuationwhich, together with other parameters of the tube, affords advantageswith respect to known devices.

According to the invention, in a device including a travelling wave tubewhich is provided with attenuating material on or near the helical delayline so that portions at both ends are free from attenuating material,the attenuation per unit length is constant, apart from the requiredtransitions at the input and output ends, while the free portion at theoutput end has a length at least ten times the wavelength in the wholeworking range of the device and the value of the product QC for the tubeis at least 0.4.

According to the invention, to reduce the length of the tube as far "aspossible, it is preferable to give the attenuation a maximum value perunit length, Whilst as a matter of fact allowance has to be made for theknown re- "ice quirement that the concentrated attenuation must exceedthe net amplification of the tube :by at least 20 db.

Due to the comparatively large distance of the attenuation from theoutput of the tube, it is primarily achieved that the output power ofthe tube becomes considerably higher than in the case where theattenuation would be nearer to the output. A second, very importantadvantage is that, due to the combination of the comparatively greatattenuation-free length at the ouptut and the selected minimum value forthe product QC, the amplitude-phase distortion, the A.M.P.M. conversionof the signal, is considerably limited with respect to known tubes anddevices.

The latter may perhaps be made acceptable as follows: Two phenomenadetermine the nonlinear behavior of a travelling wave tube. Firstly thevelocity of the electrons decreases towards the output due to energygiven off by the beam to the field of the delay line and, secondly, thefundamental harmonic OLf the density modulation in the beam is decreaseddue to the occurrence of higher harmonics. The first phenomenon impliesan increase in phase length of the delay line. The second phenomenonbecomes manifest in a decrease in phase length of the delay line, butonly of the nonattenuated portion of the output end. Thus the effect ofthe repulsion of electrons increases with the length of this portion sothat the said effect may compensate for the influence of the decrease invelocity of the beam for tubes having comparatively strong space-chargefields (high value for QC).

It is to be noted that high values for QC up to 1.0 are included incalculations, it is true, but that the practical values lie in thevicinity of 0.3. Also output portions of delay lines which are free.from attenuating material and longer than 10 wavelengths are notimpossible in known constructions. However, the combination of a high QCand a large free length with the consequent advantages is not mentionedin literature.

In this connection it is to be noted that no attention or hardly anyattention has been paid to the A.M.P.M. conversion at least inliterature and in patent specifications. Still the A.M.P.M. conversionof a travelling wave tube is important. Such tubes are frequently usedas output tubes in microwave connections. The amplitude amplification ofthe signal may become dependent upon frequency if the adjustment of thefilters in the amplifier, in front of the output tube, is incorrect, forexample, due to variation. Such amplitude variation over the frequencyrange becomes manifest, due to the A.M.P.M. conversion of the travellingwave tube, as an interfering modulation in the frequency-modulatedsignal if the A.M.- P.M. conversion exceeds a given value. Consequentlybuilders of transmitters impose certain requirements in this regard.

The invention will now be described, in detaiL'by way of example, withreference to the accompanying diagrammatic drawings, in which:

FIGURE 1 is an elevational view of a helical delay line of a tubeaccording to the invention, with attenuating material provided on theceramic carriers;

FIGURE 2 shows several attenuation profiles for this tube;

FIGURE 3 illustrates the results of measurements carried out on suchtubes;

FIGURE 4 illustrates the manner in which the output power of a tube ofFIGURE 1 varies as a function of frequency dependent on the free outputportion, and

FIGURE 5 illustrates for the same arrangement the output power as afunction of the input power and also the amplitude-phase distortion.

In FIGURE 1, a helically wound delay line 1 is a molybdenum wire of0.250 mm. thick, having a diameter of 2.2 mm. and a pitch of 0.54 mm.,while the wound length between holders 2 at the gun side and holders 3at the collector side is 130 mm. The line 1 is supported by three smallceramic rods 4 which fit into the widened ends of the holders 2, 3 andare held together by metal bands 5. A graphite layer 6, which forms theattenuating material of the tube, is sprayed from a colloidal suspensiononto the assembly comprising the helical line and the ceramic rods.

FIGURE 2 illustrates the variation in the height of the attenuationalong the delay line in db per centimeter in which the origin for thelongitudinal axis lies at the cathode side, that is to say curve A .fora conventional tube and curve B for a tube according to the invention.The tube is intended for the frequency range between 3800 rnc./s. and5000 mc./s. so that, as can be seen from the figure, in the case ofcurve B the free output portion has a length of 10 wavelengths in thewhole working range of the tube. The total attenuation is in each casesubstantially the same and is about 70 db. The tube is intended for beamaccelerating voltage between 1050 volts and 1200 volts at a beam currentof 50 mamps whereby a maximum amplification of approximately 45 db atlow level is obtained. The QC value calculated for this tube is 0.43.

In FIGURE 3 the output power of a tube W in watts is plotted along thevertical axis and the input power W in milliwatts is plotted along thehorizontal axis. The dashed curves indicated by 1050A, 1100A, and 1200Aillustrate the manner in which the output power varies as a function ofthe input power for a tube having an attenuation profile as shown byline A in FIGURE 2, at accelerating voltages of 1050 volts, etc. Thedot-anddash curves 1050B, etc., illustrate the manner in which theoutput power at these voltages varies for a tube having a dampingprofile as shown by curve B in FIGURE 2. The set of curves indicated by1050A, etc., and the set of curves indicated by 1050B, etc., haveenvelopes W and W respectively, which lines indicate the output power ata given input power for an optimum accelerating voltage adjusted between1050 and 1200 volts. From the figure it can be seen that in the case ofcurve B the output power is higher by a factor of approximately 1.5 thanin the case of curve A. The figure also shows the aligned straightlines. K and M and the inclined straight line L. In the case of curve Aand to the left of the straight lines L and K the amplitude-phasedistortion, the A.M.P.M. conversion, is smaller than 3 per db whereas inthe case of curve B the value smaller than 3 per db to the left of thelines K and M. The 3 per db limit for the A.M.P.M. conversion is apractical limit below the value of 5 /db which is adhered to by manybuilders of transmitters for the usability of a tube adjustment. Fromthe figure it appears that in the case of curve B the tube may be givenan arbitrary adjustment at an input power which is limited to 2milliwatts, whereas in the case of curve A this range is much morelimited and especially the output power may be smaller by a factor up to2. The amplification may be read from the numbers writteen with theinclined lines. All'the measurements hold good for the frequency of 4000mc./s. I

In FIGURE 4, the lines I, II, III and IVillustrate th output power W inwatts of the tube as a function of the frequency in kmc/s with anattenuation profile according to Him FIGURE 1 if the free output portionvaried from 47 mm. in line I, from 37 mm. in line II, from 27 mm. inline III and from 17 mm. in line IV. It will readily be seen that 6 highoutput power which is substantially independent of frequency is obtainedonly in the case of curve I. i

In FIGURE 5 are plotted, on the left-hand vertical axis, the outputpower in watts and, on the right-hand vertical axis, the A.M.P.M.conversion in degrees per db as a function of the input power inmilliwatts, plotted on the horizontal axis, all this at a frequency of4000 mc./s. The dot-and-dash line illustrates the output power at theoptimum accelerating voltage adjusted, the drawn. out line illustratesthe A.M.P.M. conversion at an accelerating voltage of 1060 volts, andthe dashed line illustrates the A.M.P.M. conversion for an acceleratingvoltage of 1100 volts. FIGURES 5-1 to 5-IV correspond, as to thelocation of the damping, to curves I to IV of FIGURE 4. A striking factis the low value of the A.M.- P.M. conversion in the case I, namely lessthan 2 degrees db up to saturation.

What is claimed is:

1. An electron discharge device for amplifying an electrical signal withreduced amplitude distortion for given power output comprising means toproject an electron beam along a given axis and a slow-wave propagatingstructure 'of substantially tubular form coaxial therewith andsurrounding the electron beam, said slow-wave structure comprising ahelical delay line having a central portion covered with an attenuatingmaterial and the ends of which are free of attenuating material, theportion of the delay line covered with attenuating material having adamping per unit length which is constant, the ends free of attenuatingmaterial having a length of at least 10 wavelengths for the entireoperating range of the device, said device having a value of the productQC of at least 0.4, Q being a space-charge parameter and C a gainfactor.

2. An electron discharge device as claimed in claim 1 in which theattenuating material is graphite.

References Cited UNITED STATES PATENTS 2,636,948 9/1953 Pierce 315-16 X2,771,565 11/1956 Bryant et al. 315--3.5 2,790,926 9/1957 Morton 3l53.53,005,126 10/1961 Cutler 31539.3 X

HERMAN KARL SAALBACH, Primary Examiner.

S. CHATMON, ]R., Assistant Examiner.

